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Physical Properties of Aquifers
Groundwater Hydraulics Daene C. McKinney
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Summary Occurrence of Groundwater Porous Medium Aquifer Types
Distribution of water in subsurface Porous Medium Porosity Moisture Content Particle Size Capillary Pressure Soil Moisture Characteristic Curves Specific Yield and Retention Aquifer Types Aqufier Storage Piezometric head
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Occurrence of Groundwater
Ground water occurs when water recharges the subsurface through cracks and pores in soil and rock Shallow water level is called the water table
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Distribution of Water in Subsurface
Moisture Profile Soil Profile Description Different zones depend on % of pore space filled with water Unsaturated Zone Water held by capillary forces, water content near field capacity except during infiltration Soil zone Water moves down (up) during infiltration (evaporation) Capillary fringe Saturated ar base Field capacity at top Saturated Zone Fully saturated pores Field capacity - Water remaining after gravity drainage Wilting point - Water remaining after gravity drainage & evapotranspiration
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Porous Medium Groundwater All waters found beneath the ground surface
Occupies pores (void space space not occupied by solid matter) Porous media Numerous pores of small size Pores contain fluids (e.g., water and air) Pores act as conduits for flow of fluids Type of rocks and their Number, size, and arrangement of pores Affect the storage and flow through a formation. Pores shapes are irregular Differences in the minerals making up the rocks Geologic processes experienced by them.
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Particle Size of Some Soils
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Continuum Approach to Porous Media
Pressure, density etc. apply to fluid elements that are large relative to molecular dimensions, but small relative to the size of the flow problem We adopt a Representative Elementary Volume (REV) approach REV must be large enough to contain enough pores to define the average value of the variable in the fluid phase and to ensure that the pore-to-pore fluctuations are smoothed out REV must be small enough that larger scale heterogeneities do not get averaged out (layering, etc.)
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Porosity solid Pore with water Soil volume V (Saturated)
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Porosity Property of the voids of the porous medium
solid Pore with water Soil volume V (Saturated) Property of the voids of the porous medium % of total volume occupied by voids Rhombo Packing Cubic Packing
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Porosity Porosity: total volume of soil that can be filled with water
V = Total volume of element Vi = Volume of Pores Vs = Volume of solids solid Pore with water Soil volume V (Saturated) rm = particles density (grain density) rd = bulk density Void Ratio:
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Typical Values of Porosity
Material Porosity (%) Peat Soil 60-80 Soils 50-60 Clay 45-55 Silt 40-50 Med. to Coarse Sand 35-40 Uniform Sand 30-40 Fine to Med Sand 30-35 Gravel Gravel and Sand Sandstone 10-20 Shale 1-10 Limestone
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Flow of Immiscible Fluids
Miscible displacement - fluids are completely soluble in each other, the interfacial tension between the fluids is zero, the fluids dissolve in each other, and a distinct fluid-fluid interface does not exist Immiscible displacement - simultaneous flow of immiscible fluids or phases in the porous medium. The interfacial tension between the fluids is not aero, distinct fluid-fluid interfaces exist and separate the phases in each pore. Unsaturated flow - flow of two immiscible fluids (water and air), except that the air is practically immobile.
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Saturation Saturation Water Content Water Saturation Soil volume V
(Unsaturated) Saturation Water Content Water Saturation
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Particle Size Distribution
Poorly sorted silty fine to medium sand Well sorted fine sand Particle size distribution curves Relative % of grain sizes Soil classification standards Soil texture
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Particle Size Distribution
Sand 49% Clay 40% Soil Characteristics of Cyprus Soil Sample
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Surface Tension air water Below interface At interface
Forces act equally in all directions At interface Some forces are missing Pulls molecules down and together Like membrane exerting tension on the surface Curved interface Higher pressure on concave side Pressure increase is balanced by surface tension s = N/m 20oC) Capillary pressure Relates pressure on both sides of interface water air No net force Net force inward Interface
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Surface Tension sgl ssg b < 90o - liquid is wetting the solid
gas liquid ssg b < 90o - liquid is wetting the solid b > 90o - liquid is non-wetting the solid solid ssl Hg solid air b water solid air b Mercury nonwetting solid Water wetting solid
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Capillary Pressure Two immiscible fluids in contact exhibit a discontinuity in pressure across the interface separating them. This pressure difference is capillary pressure pc It depends on the curvature of the interface. pnw is the pressure in the nonwetting fluid (air, say) pw is the pressure in the wetting fluid (water, say)
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Capillary Pressure Solid Water Air r Rise of water in a capillary tube. Capillary forces must balance the weight of water Capillary pressure head
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Capillary Pressure B A (A) Below the water level
Solid Water Air r Negative pressure Positive B A (A) Below the water level (B) Above the water level Difference in pressure across the interface is
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Drainage Drainage occurs when the water pressure in the pores becomes less than the air pressure Interfacial tension prevents displacement of water in the left pore r solid Pore water press. = -p Pore air press. = 0 If pc increases, radius must decrease, or water occupies smaller pores. Water recedes into pores small enough to support the interface with a radius required to balance the capillary force. Water drains from the large pores first.
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Energy in Flow Systems Velocity head p/g Pressure head z
v2/(2g) Velocity head p/g Pressure head z Elevation head v2/(2g) EGL p/g HGL v2/(2g) z Hydraulic grade line (HGL) – height of water in piezometer tube datum Energy grade line (EGL) – Height of water in pitot tube
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Piezometric Head Confined aquifer Unconfined aquifer Pressure head = 0
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Piezometric Head in Unsaturated Flow
Soil volume V (Unsaturated) Saturated Zone Water Table Unsaturated Zone q = f q < f < 0 = 0 > 0 pw > 0 pw = 0 pw < 0
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Subsurface Pressure Distribution
Capillary pressure head in zone above water table Hydrostatic pressure distribution exists below the water table (p = 0). Ground surface Pressure is negative above water table Unsaturated zone Water table Pressure is positive below water table Saturated zone
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Soil Water Characteristic Curves
y Vadose Zone Porosity Capillary Zone yb Critacal Head (Bubbling Press.) qo f Capillary pressure head Function of: Pore size distribution Moisture content Irreducible Water content Porosity
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Capillary Rise in Soils
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Aquifer Types Confined aquifer Unconfined aquifer Aquifer Aquitard
Under pressure Bounded by impervious layers Unconfined aquifer Phreatic or water table Bounded by a water table Aquifer Store & transmit water Unconsolidated deposits sand and gravel, sandstones etc. Aquitard Transmit don’t store water Shales and clay
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Summary Occurrence of Groundwater Porous Medium Aquifer Types
Distribution of water in subsurface Porous Medium Porosity Moisture Content Particle Size Capillary Pressure Soil Moisture Characteristic Curves Specific Yield and Retention Aquifer Types Aqufier Storage Piezometric head
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